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<H1>symmetric(+Graph)</H1>
Ensures Graph is symmetric
<DL>
<DT><EM>Graph</EM></DT>
<DD>A graph.
</DD>
</DL>
<H2>Description</H2>
Ensures that for every edge (x,y) added to the Graph the symmetric edge (y,x) is also added.<br />
					Ensures that for every edge (x,y) removed from the Graph the symmetric edge (y,x) is also removed.
<H3>Fail Conditions</H3>
Fails 
			 if Graph is not a graph variable or
			 if Graph can not be enforced to be symmetric.
			
<H2>Examples</H2>
<PRE>
?- symmetric(G).
No.
			 
?- V`::[]..[1,2,3], E`::[[2,3]]..[[1,2],[1,3],[2,1],[2,3],[3,1]], dirgraph(G,V,E), symmetric(G).
No.
			 
?- V`::[]..[1,2,3], E`::[]..[[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]], dirgraph(G,V,E), symmetric(G).
V = V{cardinal([[]:0, [1, 2, 3]:3], Card{cardinal : _628, fd:[0..3]}, _520, _521, _522, [], [], ['SUSP-_2494-susp'], ['SUSP-_2104-dead'])}
E = E{cardinal([[]:0, [[1, 2], [1, 3], [2, 1], [3, 1]]:4], Card{cardinal : _829, fd:[0..4]}, _721, _722, _723, [], ['SUSP-_3338-susp', 'SUSP-_2504-susp'], ['SUSP-_3347-susp'], ['SUSP-_3248-dead'])}
G = dirgraph(V{cardinal([[]:0, [1, 2, 3]:3], Card{cardinal : _628, fd:[0..3]}, _520, _521, _522, [], [], ['SUSP-_2494-susp'], ['SUSP-_2104-dead'])}, E{cardinal([[]:0, [[1, 2], [1, 3], [2, 1], [3, 1]]:4], Card{cardinal : _829, fd:[0..4]}, _721, _722, _723, [], ['SUSP-_3338-susp', 'SUSP-_2504-susp'], ['SUSP-_3347-susp'], ['SUSP-_3248-dead'])})
 
?- V`::[]..[1,2,3], E`::[[1,2]]..[[1,2],[1,3],[2,1],[2,3],[3,1],[3,2]], dirgraph(G,V,E), symmetric(G).
V = V{cardinal([[1, 2]:2, [3]:3], Card{cardinal : _688, fd:[2, 3]}, _580, _581, _582, [], [], ['SUSP-_3503-dead', 'SUSP-_2633-susp'], ['SUSP-_3584-dead'])}
E = E{cardinal([[[1, 2], [2, 1]]:2, [[1, 3], [2, 3], [3, 1], [3, 2]]:6], Card{cardinal : _902, fd:[2..6]}, _794, _795, _796, [], ['SUSP-_4193-susp', 'SUSP-_2643-susp'], ['SUSP-_4202-susp'], ['SUSP-_4109-dead'])}
G = dirgraph(V{cardinal([[1, 2]:2, [3]:3], Card{cardinal : _688, fd:[2, 3]}, _580, _581, _582, [], [], ['SUSP-_3503-dead', 'SUSP-_2633-susp'], ['SUSP-_3584-dead'])}, E{cardinal([[[1, 2], [2, 1]]:2, [[1, 3], [2, 3], [3, 1], [3, 2]]:6], Card{cardinal : _902, fd:[2..6]}, _794, _795, _796, [], ['SUSP-_4193-susp', 'SUSP-_2643-susp'], ['SUSP-_4202-susp'], ['SUSP-_4109-dead'])})
 
?- V`::[]..[1,2,3], E`::[[1,2]]..[[1,2],[1,3],[2,1],[2,3],[3,1]], dirgraph(G,V,E), symmetric(G).
V = V{cardinal([[1, 2]:2, [3]:3], Card{cardinal : _658, fd:[2, 3]}, _550, _551, _552, [], [], ['SUSP-_3426-dead', 'SUSP-_2556-susp'], ['SUSP-_3507-dead'])}
E = E{cardinal([[[1, 2], [2, 1]]:2, [[1, 3], [3, 1]]:4], Card{cardinal : _864, fd:[2..4]}, _756, _757, _758, [], ['SUSP-_4162-susp', 'SUSP-_2566-susp'], ['SUSP-_4171-susp'], ['SUSP-_4052-dead'])}
G = dirgraph(V{cardinal([[1, 2]:2, [3]:3], Card{cardinal : _658, fd:[2, 3]}, _550, _551, _552, [], [], ['SUSP-_3426-dead', 'SUSP-_2556-susp'], ['SUSP-_3507-dead'])}, E{cardinal([[[1, 2], [2, 1]]:2, [[1, 3], [3, 1]]:4], Card{cardinal : _864, fd:[2..4]}, _756, _757, _758, [], ['SUSP-_4162-susp', 'SUSP-_2566-susp'], ['SUSP-_4171-susp'], ['SUSP-_4052-dead'])})
			</PRE>

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